Small Craft Advisory 716, NWS Anchorage, AK

Southcentral and Southwest Alaska Forecast Discussion
National Weather Service Anchorage AK
402 PM AKDT Mon Sep 21 2020

.ANALYSIS AND UPPER LEVELS...

This afternoon, a near full latitude trough extended in a negative
tilt from eastern Siberia, through the Alaska Peninsula, into the
North Pacific. Several pieces of energy are seen in the satellite
data diving into this trough from the north and then rotating
around the low. This is helping foster scattered shower
development across the Alaska Peninsula, Southwest Alaska, and
into the Gulf and Southcentral.

&&

.MODEL DISCUSSION...

Today's numerical runs show good agreement at the surface and
aloft with the low transitioning from the Southern Bering into the
Gulf, but then differences arise in the 60-84 hour period as
additional shortwave energy aloft dives into the backside of the
trough the parent low is currently in. As this additional energy
arrives, new surface cyclogenesis commences and pivot in a
counter clockwise fashion, as the two lows rotate around one
another (Fujiwhara effect).

At what strength and what location remains the question, as the
Canadian is deepest at 977 mb by day 3 (Thursday) just outside of
Price William Sound, and the GFS weakest/furthest south across the
Southern Gulf. The NAM and ECMWF are in between these models in
position and strength. This adds some uncertainty to wind field
orientation, strength, and precipitation amounts.

&&

.AVIATION...

PANC...Light and variable winds will prevail at the terminal
through the forecast period. The main challenge through, as low
stratus from the Cook Inlet moved overhead earlier, and it's still
lurking around with it being reported at 500 feet in a scattered
variety. How long this will continue is the main question at this
time.

&&

.SHORT TERM FORECAST SOUTHCENTRAL ALASKA (Days 1 through 3:
Monday night through Thursday)...

A frontal system will continue to move north from Kodiak Island and
across the Northern Gulf, bringing Gale to Storm force winds through
Wednesday. Otherwise, a series of lows will rotate across the Gulf
and past Kodiak Island, keeping coastal areas quite wet through the
period. Expect as much as 2 to 4 inches of rain from western Prince
William Sound to Kodiak Island. Some rain will spill over the
Chugach Mtns into parts of the southern Copper River Basin, but most
inland areas of Southcentral will generally be dry with a few
showers possible this evening.

The main upper trough and low will begin to exit eastward on
Wednesday, which will allow winds to diminish across the Gulf.
However, it does look like one more surface low will develop and
track westward across the central Gulf. There is quite a bit of
uncertainty in the track and strength of this low, but it does look
like it will stay out over the Gulf. Some of the moisture with this
low will advect toward Southcentral, so expect increasing chances of
rain for inland areas Wednesday through Thursday.

&&

.SHORT TERM FORECAST SOUTHWEST ALASKA (Days 1 through 3)...

Gusty northeasterly winds will gradually turn more northerly through
mid-week with both high and low temperatures trending downward to
the mid 40s and mid 30s, respectively. While a few showers are
possible along the immediate Kuskokwim Delta coast and near the
Alaska Range, much of the area should remain dry through Thursday in
the cool, dry northerly flow.

&&

.SHORT TERM FORECAST BERING SEA/ALEUTIANS (Days 1 through 3)...

A cool, northwesterly flow will persist through Wednesday,
generally east of Unalaska between a low in the Gulf and a ridge
over the western Bering. Gusty winds are expected through this
evening, particularity out of gaps on the Pacific side of the
central/eastern Aleutians. Scattered showers will persist through
Tuesday and will gradually become more isolated thereafter as the
air mass becomes drier. Winds will likewise diminish as the
aforementioned low weakens and moves farther east into the Gulf.

&&

.MARINE (Days 3 through 5/Thursday through Saturday)...

Gulf of Alaska: There is moderate confidence that the broad low
pressure system in the western Gulf will break down through Friday
afternoon. The GFS deterministic model has the low remaining in
the Gulf longer than other models but winds and waves should
remain at or below small craft criteria through Friday. On
Saturday there is moderate to high confidence that a Gale force
low will move into the Gulf from the North Pacific. Model spread
of the low center pressure ranges from 975 mb to 986 mb. There is
low confidence in the location of maximum winds at this time due
to high model spread on the east/west location of the low center.

Bering Sea/Aleutian Chain: There is high confidence that winds and
waves will be mostly calm through the region on Thursday. On
Friday morning a Polar trough will bring moderate (<34 kts)
northerly winds through the Bering Strait and into the Bering Sea
by Friday evening. On Saturday a weak northerly shortwave and
surface low may create winds up to gale force over the western
Bering. Confidence in winds reaching gale force on Saturday is
moderate.

&&

.LONG TERM FORECAST (Days 4 through 7/Friday through Monday)...

Synoptically, a broad longwave trough is expected to persist over
the region with the jet centered between the 40th and 50th
parallel. This should continue the fall pattern of rainy weather
for most of the region, but primarily the Bering Sea, AKPEN and
Gulf coast. Late Friday and Saturday will likely be the wettest
for the Alaska Mainland as a moderately strong storm moves into
the Gulf. The broad nature of this system, extending from Bristol
Bay to Southeast Alaska will allow for upper level shortwaves to
bring moisture and rain into the Alaska interior. Model guidance
has come to better agreement in regards to this low, with the low
center moving into the Prince William Sound Sunday morning. By
Monday, model guidance begins to considerably differ once again,
with the GFS bringing the storm back into the Gulf and the EC
pushing it into Bristol Bay.

Northerly flow on the backside of the aforementioned low is
expected to bring a polar airmass over the Bering on Friday. A
series of shortwaves may help provide lift needed for showers
over the western Bering and Aleutian Chain through the forecast
period. Simultaneous ridging over the Kamchatka Peninsula will
help to bring warmer air in from the Western Pacific, thus setting
the stage for the next baroclinic storm in the Western Bering.
Models are in excellent agreement that a Kamchatka low will form
Monday morning with the front reaching the Western Aleutians on
Monday afternoon.

&&

.AFC WATCHES/WARNINGS/ADVISORIES...
PUBLIC...NONE.
MARINE...Storm Warning 119 120 131.
Gale Warning 130 138 139 150 155 170 171 172 174 351.
FIRE WEATHER...NONE.

&&

SYNOPSIS AND MODEL DISCUSSION...PD
SOUTHCENTRAL ALASKA...RC
SOUTHWEST ALASKA/BERING SEA/ALEUTIANS...AP
MARINE/LONG TERM...CJ